Fuel injection systems of the latest generation usually work on the common rail principle and often contain injectors driven piezo-electrically. Here one or more such piezo injectors, which can be opened and closed in a targeted manner, are provided at each combustion chamber of the internal combustion engine. When the injectors are open, fuel enters the interior of the combustion chamber and combusts there. To ensure good combustion and exhaust emissions, and for comfort reasons, the injected fuel quantity should be determined as precisely as possible.
WO 2009/010374 A1 discloses a method and a device for forming an electrical control signal for an injection pulse of a fuel injector. This electrical control signal activates a piezo-electric actuator to inject a predefined fuel quantity into a cylinder of an internal combustion engine. Using the curve of the electrical control signal, an injection rate of the fuel injector is regulated as a function in particular of the rail pressure, the stroke travel and/or the opening duration of the fuel injector. For at least a partial fuel quantity to be injected, the curve of the electrical control signal can be freely formed in relation to at least one pulse flank and/or amplitude. The form of the injection pulse is structured such that the predefined fuel quantity for injection is held constant irrespective of the curve of the electrical control signal.
When forming the rate curve for the fuel, it is important to maintain the injection quantities required by the internal combustion engine for mixture formation within tight tolerances in order to influence the emissions and fuel consumption of the respective motor vehicle in the desired manner.
One essential aspect in the forming of the rate curve is the so-called part-stroke operation. Here the nozzle needle is held in a middle position between the nozzle seat (injector closed) and the end stroke position (injector opened to the maximum) of the nozzle needle in order to influence the fuel flow through the nozzle and hence the mixture formation.
In practice there is a problem in setting and achieving the said part stroke precisely, in that the injection quantity required by the internal combustion engine can be guaranteed as an integral of the fuel flow through the nozzle, which is dependent on the injection nozzle needle stroke. This problem arises because in part-stroke operation, component tolerances of the injector under different ambient conditions (pressure, temperature) in operation of the injector in an internal combustion engine, because of the steepness of the flow curve of the nozzle, over the needle stroke, have a tendentially greater effect than is the case in full-stroke operation of the injector.
In internal combustion engines, the benefits of forming the rate curve and its influence on emissions have been primarily studied on internal combustion engines in which the cylinder pressure, various temperatures and sometimes also the needle stroke are monitored by means of external sensors. Use of such sensors is costly and is not therefore applied in motor vehicles for cost reasons.